1. Field of the Invention
The invention relates to radiation imaging and more particularly to phototimers for detecting and automatically controlling patient exposure to radiation.
2. Background Art
Phototimers of the type having one or more photosensors positioned behind a subject in the path of an X-ray beam to control the X-ray exposure of the subject are well known. U.S. Pat. No. 4,748,649 issued to Griesmer et al. on May 31, 1988 shows a phototimer having three photosensors in a triangular arrangement. Depending upon the diagnostic procedure being performed, the operator selects any one or any combination of the outputs from the three sensors, which are then combined and compared to a computer generated reference level to control the X-ray exposure. Proper exposure depends upon correct placement of the phototimer sensors with respect to the patient. Typically, for a chest radiograph, the output from a pair of the photosensors is chosen. The phototimer is positioned with respect to the patient such that the two sensors of the pair are positioned on either side of the midline in the upper lung fields. It is often the case particularly in bedside radiography, where a film and phototimer are slipped under the patient to perform the exposure, that the sensors are not properly located with respect to the patient, resulting in an incorrect exposure. Also, where a patient is missing one lung or one lung is filled with fluid, an incorrect exposure is achieved. The incorrect exposure is discovered only upon developing the film. In 5 to 10 percent of the bedside radiographs, the exposure is so poor as to necessitate repeating the procedure.
It is the object of the present invention to provide a phototimer for detecting and controlling X-ray exposures that avoids the problems noted above.